General multipoles and their implications for dark matter inference

ABSTRACT The flux ratios of strongly lensed quasars have previously been used to infer the properties of dark matter. In these analyses, it is crucial to separate the effect of the main lensing galaxy and the low-mass dark matter halo population. In this work, we investigate flux-ratio perturbations resulting from general third- and fourth-order multipole perturbations to the main lensing galaxy’s mass profile. We simulate four lens systems, each with a different lensing configuration, without multipoles. The simulated flux ratios are perturbed by 10–40 per cent by a population of low-mass haloes consistent with cold dark matter and, in one case, also a satellite galaxy. This level of perturbation is comparable to the magnitude of flux-ratio anomalies in real data that has been previously analysed. We then attempt to fit the simulated systems using multipoles instead of low-mass haloes. We find that multipoles with amplitudes of 0.01 or less can produce flux-ratio perturbations in excess of 40 per cent. In all cases, third- or fourth-order multipoles can individually reduce the magnitude of, if not eliminate, flux-ratio anomalies. When both multipole orders are jointly included, all simulated flux ratios can be fit to within the observational uncertainty. Our results indicate that low-mass haloes and multipoles are highly degenerate when modelling quadruply imaged quasars based just on image positions and flux ratios. In the presence of this degeneracy, flux-ratio anomalies in lensed quasars alone cannot be used to place strong constraints on the properties of dark matter without additional information that can inform our priors.